Method for generating pollution credits while processing reactive metals

ABSTRACT

This invention relates to a method for generating pollution credits while processing molten magnesium, aluminum, lithium, and alloys of such metals by contacting the molten metal or alloy with a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/780,256, filed Feb. 9, 2001, and now U.S. Pat. No.6,537,346, and claims priority to U.S. Provisional Patent ApplicationNo. 60/202,169, filed May 4, 2000, each of which is incorporated hereinby reference in its entirety.

FIELD OF THE INVENTION

This invention relates in one aspect to a method for generatingpollution credits while processing molten reactive metals such asmagnesium, aluminum, lithium, and alloys of such metals.

BACKGROUND OF THE INVENTION

Molded parts made of magnesium (or its alloys) are finding increasinguse as components in the automotive and aerospace industries. Theseparts are typically manufactured in a foundry, where the magnesium isheated to a molten state to a temperature as high as 1400° F. (800° C.),and the resulting molten magnesium is poured into molds or dies to formingots or castings. During this casting process, protection of themagnesium from atmospheric air is essential to prevent a spontaneousexothermic reaction from occurring between the reactive metal and theoxygen in the air. Protection from air is also necessary to minimize thepropensity of reactive magnesium vapors to sublime from the molten metalbath to cooler portions of a casting apparatus. In either situation, anextremely hot magnesium fire can result within a few seconds of airexposure, potentially causing extensive property damage and seriousinjury or loss of human life. Similarly, aluminum, lithium, and alloysof such metals are highly reactive in molten form necessitatingprotection from atmospheric air.

Various methods have been investigated to minimize the exposure ofmolten magnesium to air. See J. W. Fruehling et al., Transactions of theAmerican Foundry Society, Proceeding of the 73^(rd) Annual Meeting, May5-9, 1969, 77 (1969). The two most viable methods for effectivelyseparating molten magnesium from air are the use of salt fluxes and theuse of cover gases (sometimes referred to as “protective atmospheres”).A salt flux is fluid at the magnesium melt temperature and iteffectively forms a thin impervious film on the surface of themagnesium, thus preventing the magnesium from reacting with oxygen inthe air. However, the use of salt fluxes presents several disadvantages.First, the flux film itself can oxidize in the atmosphere to harden intoa thick deposit of complex metal oxide/chlorides, which is easilycracked to expose molten magnesium to the atmosphere. Second, the saltfluxes are typically hygroscopic and, as such, can form salt inclusionsin the metal surface which can lead to corrosion. Third, fumes and dustparticles from fluxes can cause serious corrosion problems to ferrousmetals in the foundry. Fourth, salt sludge can form in the bottom of thecrucible. Fifth, and not least, removal of such fluxes from the surfaceof cast magnesium parts can be difficult.

As a result, there has been a shift from using salt fluxes to usingcover gases to inert molten magnesium. Cover gases can be described asone of two types: inert cover gases and reactive cover gases. Inertcover gases can be non-reactive (e.g., argon or helium) or slowlyreactive (e.g., nitrogen, which reacts slowly with molten magnesium toform Mg₃N₂). For inert cover gases to be effective, air must beessentially excluded to minimize the possibility of metal ignition,i.e., the system must be essentially closed. To utilize such a closedsystem, workers either have to be equipped with a cumbersomeself-contained breathing apparatus or they have to be located outside ofthe dimensions of the processing area (e.g., by using remote control).Another limitation of inert cover gases is that they are incapable ofpreventing molten metal from subliming.

Reactive cover gases are gases used at low concentration in a carriergas, normally ambient air, that react with the molten magnesium at itssurface to produce a nearly invisible, thermodynamically stable film. Byforming such a tight film, the aerial oxygen is effectively separatedfrom the surface of the molten magnesium, thus preventing metal ignitionand minimizing metal sublimation.

The use of various reactive cover gases to protect molten magnesium fromignition has been investigated as early as the late 1920s. An atmospherecontaining CO₂ is innocuous and economical yet forms a protective filmon a magnesium surface which can prevent ignition for over 1 hour at 650° C. However, the CO₂-based films formed are dull in appearance andunstable, especially in the presence of high levels of air, andconsequently offer little protection for the magnesium surface fromambient oxygen. In effect, the CO₂ behaves more like an inert cover gasthan a reactive cover gas.

U.S. Pat. No. 4,770,697 (Zurecki) discloses the use ofdichlorodifluoromethane as a blanketing atmosphere or cover gas formolten aluminum-lithium alloys. U.S. Pat. Nos. 6,398,844 and 6,521,018(both Hobbs et al.) disclose blanketing gases used with non-ferrousmetals and alloys with reduced Global Warming Potentials, but which arevery toxic to workers and/or corrosive to process equipment.

SO₂ has been investigated in the past as a reactive cover gas, as SO₂reacts with molten magnesium to form a thin, nearly invisible film ofmagnesium oxysulfides. SO₂ is low in cost and is effective at levels ofless than 1% in air in protecting molten magnesium from ignition.However, SO₂ is very toxic and consequently requires significantmeasures to protect workers from exposure (permissible exposure levelsare only 2 ppm by volume or 5 mg/m³ by volume). Another problem with SO₂is its reactivity with water in humid air to produce very corrosiveacids (H₂SO₄ and H₂SO₃). These acids can attack unprotected workers andcasting equipment, and they also contribute significantly to acid rainpollution when vented out of the foundry. SO₂ also has a tendency toform reactive deposits with magnesium which produce metal eruptions fromthe furnace (especially when SO₂ concentrations in the air are allowedto drift too high). Though SO₂ has been used commercially on a largescale for the casting of magnesium alloys, these drawbacks have led somemanufacturers to ban its use.

Fluorine-containing reactive cover gases provide an inert atmospherewhich is normally very stable to chemical and thermal breakdown.However, such normally stable gases will decompose upon contact with amolten magnesium surface to form a thin, thermodynamically stablemagnesium oxide/fluoride protective film. U.S. Pat. No. 1,972,317(Reimers et. al.) describes the use of fluorine-containing compoundswhich boil, sublime or decompose at temperatures below about 750° C. toproduce a fluorine-containing atmosphere which inhibits the oxidation ofmolten magnesium. Suitable compounds listed include gases, liquids orsolids such as BF₃, NF₃, SiF₄, PF₅, SF₆, SO₂F₂, (CClF₂)₂, HF, NH₄F andNH₄PF₆. The use of BF₃, SF₆, CF₄ and (CClF₂)₂ as fluorine-containingreactive cover gases is disclosed in J. W. Fruehling et al., describedsupra.

Each of these fluorine-containing compounds has one or moredeficiencies. Though used commercially and effectively at lower levelsthan SO₂, BF₃ is toxic and corrosive and can be potentially explosivewith molten magnesium. NF₃, SiF₄, PF₅, SO₂F₂ and HF are also toxic andcorrosive. NH₄F and NH₄PF₆ are solids which sublime upon heating to formtoxic and corrosive vapors. CF₄ has a very long atmospheric lifetime.(CClF₂)₂, a chlorofluorocarbon, has a very high ozone depletionpotential (ODP). The ODP of a compound is usually defined as the totalsteady-state ozone destruction, vertically integrated over thestratosphere, resulting from the unit mass emission of that compoundrelative to that for a unit mass emission of CFC-11 (CCl₃F). SeeSeinfeld, J. H. and S. N. Pandis, Atmospheric Chemistry and Physics:From Air Pollution to Climate Change, John Wiley & Sons, Inc., New York,(1998). Currently, there are efforts underway to phase out theproduction of substances that have high ODPs, includingchlorofluorocarbons and HCFCs, in accordance with the Montreal Protocol.UNEP (United Nations Environment Programme), Montreal Protocol onSubstances that Deplete the Ozone Layer and its attendant amendments,Nairobi, Kenya, (1987).

Until recently, SF₆ was considered the optimum reactive cover gas formagnesium. SF₆ is effective yet safe (essentially inert, odorless, lowin toxicity, nonflammable and not corrosive to equipment). It can beused effectively at low concentrations either in air (<1%) or in CO₂ toform a very thin film of magnesium oxyfluorides and oxysulfides on thesurface of molten magnesium. This magnesiumoxide/fluoride/sulfide/sulfur oxide film is far superior at protectingthe magnesium from a vigorous exothermic oxidation reaction than is themagnesium oxide film inherently present on the metal surface. Themagnesium oxide/fluoride/sulfide/sulfur oxide film is sufficiently thin(i.e., nearly invisible to the naked eye) that the metal surface appearsto be metallic. This superior protection is believed to result from thegreater thermodynamic stability of a nonporous magnesium sulfide/sulfuroxide and/or magnesium oxide/fluoride film as compared to the stabilityof a thick porous film of either magnesium oxide, sulfide or fluoridealone.

In a typical molten magnesium process employing a reactive cover gas,only a small portion of the gas passed over the molten magnesium isactually consumed to form that film, with the remaining gas beingexhausted to the atmosphere. Efforts to capture and recycle the excessSF₆ are difficult and expensive due to its very low concentrations inthe high volumes of exhaust stream. Efficient thermal oxidizingequipment would be required to remove the SF₆ from the exhaust stream,adding significantly to production costs. Product costs can also beconsiderable, as SF₆ is the most expensive commercially used reactivecover gas.

However, perhaps the greatest concern with SF₆ is its very significantglobal warming potential (3200 year atmospheric lifetime, and about22,200 times the global warming potential of carbon dioxide). At theDecember 1997 Kyoto Summit in Japan, representatives from 160 countriesdrafted a legally binding agreement containing limits for greenhouse gasemissions. The agreement covers six gases, including SF₆, and includes acommitment to lower the total emissions of these gases by the year 2010to levels 5.2% below their total emissions in 1990. UNEP (United NationsEnvironment Programme), Kyoto Protocol to the United Nations FrameworkConvention on Climate Change, Nairobi, Kenya, 1997.

As no new replacement for SF₆ is yet commercially available, efforts areunderway to reinvigorate SO₂, as SO₂ has essentially no global warmingpotential (despite its other considerable drawbacks). See H. Gjestland,P. Bakke, H. Westengen, and D. Magers, Gas protection of moltenmagnesium alloys: SO₂ as a replacement for SF₆. Presented at conferenceon Metallurgie du Magnesium et Recherche d'Allegement dans I''Industriedes Transports, International Magnesium Association (IMA) and Pole deRecherche et de Devleoppment Industriel du Magnesium (PREDIMAG)Clermond-Ferrand, France, October 1996.

The data in TABLE 1 summarize selected safety and environmentallimitations of compounds currently known to be useful in the protectionof molten magnesium. Numbers followed by an asterisk (*) areparticularly problematic with regard to safety and/or environmentaleffects.

TABLE 1 Global Ozone Atmo- Warming Depletion spheric Potential-Potential- Com- Exposure Lifetime⁽³⁾ GWP⁽³⁾ ODP⁽³⁾ pound Guideline⁽¹⁾(yrs) (100 yr ITH) (CFC-11 = 1) SO₂ 2 ppmv* BF₃ 1 ppmv* NF₃ 10 ppmv* 740 10800* SiF₄ 2.5 mg/m³ as F* PF₅ 2.5 mg/m³ as F* SF₆ 1000 ppmv 320022200* SO₂F₂ 5 ppmv* (CClF₂)₂ 1000 ppmv 300 9800* 0.85* HF 3 ppmvceiling* NH₄F 2.5 mg/m³ as F* NH₄PF₆ corrosive, causes burns⁽²⁾* CF₄Moderately toxic 50000* 5700* by inhalation CHClF₂ 1000 ppmv 11.8 1900*0.055* ⁽¹⁾The Condensed Chemical Dictionary, edited by Gessner G.Hawley. New York, Van Nostrand Reinhold Co. (1981). Note: ppmv = partsper million by volume. ⁽²⁾Material Safety Data Sheet for ammoniumhexafluorophosphate, Sigma-Aldrich Corporation, Milwaukee, WI. ⁽³⁾WorldMeterological Organization Global Research and Monitoring Project-ReportNo.44, “Scientific Assessment of Ozone Depletion: 1998,” WMO (1999).

As each of these compounds presents either a significant safety or anenvironmental concern, the search continues to identify new reactivecover gases for protecting molten magnesium, aluminum, lithium, andalloys of such metals which are simultaneously effective, safe,environmentally acceptable, and cost-effective.

SUMMARY OF THE INVENTION

This invention relates in one aspect to a method for generatingpollution credits while processing molten reactive metals and alloys ofsuch metals, e.g., magnesium, aluminum, lithium, and alloys of one ormore of such metals. Reactive metals are metals (and alloys) which aresensitive to destructive, vigorous oxidation in air. In brief summary,the invention provides a method for generating pollution creditscomprising:

(a) treating molten reactive metal or alloy of such metal to protectsaid metal or alloy from reacting with oxygen in air by (1) providingmolten metal or alloy and (2) exposing said metal or alloy to a gaseousmixture comprising a fluorocarbon selected from the group consisting ofperfluoroketones, hydrofluoroketones, and mixtures thereof to yieldprotected metal or alloy having a protective film thereon; and

(b) taking allocation of pollution credits.

In one embodiment, this invention employs a method for treating moltenreactive metal or alloy to protect it from reacting with oxygen in air.The method comprises providing molten reactive metal or alloy andexposing it to a gaseous mixture comprising a fluorocarbon selected fromthe group consisting of perfluoroketones, hydrofluoroketones, andmixtures thereof. The gaseous mixture may further comprise a carriergas. The carrier gas may be selected from the group consisting of air,carbon dioxide, argon, nitrogen and mixtures thereof.

One advantage of the present invention over the known art is that theGlobal Warming Potentials of perfluoroketones and hydrofluoroketones arequite low. Therefore, the present inventive process is moreenvironmentally friendly. By employing the method for treating orprotecting molten reactive metals or alloys which is described herein,processors who handle molten reactive metals or alloys will be able toproduce unit quantities of such metals and alloys and parts containingsuch metals and alloys as before while generating much smallerquantities of materials exhibiting significant GWP contribution or otherenvironmentally desirable effect.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

Fluorocarbons used in the present invention include perfluoroketones(PFKs), and hydrofluoroketones (HFKs) which incorporate limited amountsof hydrogen in their structures. These fluorocarbons can be effective asreactive cover gases to protect reactive molten reactive metals such asmolten magnesium from ignition. As is the case with knownfluorine-containing reactive cover gases, these fluorocarbons can reactwith the molten metal surface to produce a protective surface film, thuspreventing ignition of the molten metal. For convenience, the followingdescription refers to molten magnesium, but it should be understood thatthe invention is also applicable to other reactive molten metals andalloys, including aluminum, lithium and alloys of one or more ofmagnesium, aluminum or lithium.

For the protection of molten magnesium from ignition, fluorocarbons ofthe present invention are desirable alternatives to the most commonlyused cover gas currently, SF₆. The fluorocarbons of the presentinvention are low GWP fluorocarbon alternatives to SF₆, i.e., thefluorocarbons of the present invention have measurably lower globalwarming potential relative to SF₆ (i.e., significantly less than 22,200)and are not significantly worse in atmospheric lifetime, ozone depletionpotential, or toxicity properties.

Perfluorinated ketones (PFKs) useful in the present invention includeketones which are fully fluorinated, i.e., all of the hydrogen atoms inthe carbon backbone have been replaced with fluorine atoms. The carbonbackbone can be linear, branched, or cyclic, or combinations thereof,and will preferably have about 5 to about 9 carbon atoms. Representativeexamples of perfluorinated ketone compounds suitable for use in theprocesses and compositions of the invention include CF₃CF₂C(O)CF(CF₃)₂,(CF₃)₂CFC(O)CF(CF₃)₂, CF₃(CF₂)₂C(O)CF(CF₃)₂, CF₃(CF₂)₃C(O)CF(CF₃)₂,CF₃(CF₂)₅C(O)CF₃, CF₃CF₂C(O)CF₂CF₂CF₃, CF₃C(O)CF(CF₃)₂,perflurocyclohexanone, and mixtures thereof. In addition todemonstrating reactive cover gas performance, perfluorinated ketones canoffer additional important benefits in safety of use and inenvironmental properties. For example, CF₃CF₂C(O)CF(CF₃)₂ has low acutetoxicity, based on short-term inhalation tests with mice exposed forfour hours at a concentration of 100,000 ppm in air. Also based onphotolysis studies at 300 nm CF₃CF₂C(O)CF(CF₃)₂ has an estimatedatmospheric lifetime of 5 days. Other perfluorinated ketones showsimilar absorbances and thus are expected to have similar atmosphericlifetimes. As a result of their rapid degradation in the loweratmosphere, the perfluorinated ketones have short atmospheric lifetimesand would not be expected to contribute significantly to global warming(i.e., low global warming potentials). Perfluorinated ketones which arestraight chain or cyclic can be prepared as described in U.S. Pat. No.5,466,877 (Moore et al.) which in turn can be derived from thefluorinated esters described in U.S. Pat. No. 5,399,718 (Costello etal.). Perfluorinated ketones that are branched can be prepared asdescribed in U.S. Pat. No. 3,185,734 (Fawcett et al.). All of thesepatents are incorporated by reference in their entirety.

Hydrofluoroketones (HFKs) that are useful in the present inventioninclude those ketones having only fluorine and hydrogen atoms attachedto the carbon backbone. The carbon backbone can be linear, branched, orcyclic, or combinations thereof, and preferably will have about 4 toabout 7 carbon atoms. Representative examples of hydrofluoroketonecompounds suitable for use in the processes and compositions of thisinvention include: HCF₂CF₂C(O)CF(CF₃)₂, CF₃C(O)CH₂C(O)CF₃,C₂H₅C(O)CF(CF₃)₂, CF₂CF₂C(O)CH₃, (CF₃)₂CFC(O)CH₃, CF₃CF₂C(O)CHF₂,CF₃CF₂C(O)CH₂F, CF₃CF₂C(O)CH₂CF₃, CF₃CF₂C(O)CH₂CH₃, CF₃CF₂C(O)CH₂CHF₂,CF₃CF₂C(O)CH₂CHF₂, CF₃CF₂C(O)CH₂CH₂F, CF₃CF₂C(O)CHFCH₃,CF₃CF₂C(O)CHFCHF₂, CF₃CF₂C(O)CHFCH₂F, CF₃CF₂C(O)CF₂CH₃,CF₃CF₂C(O)CF₂CHF₂, CF₃CF₂C(O)CF₂CH₂F, (CF₃)₂CFC(O)CHF₂,(CF₃)₂CFC(O)CH₂F, CF₃CF(CH₂F)C(O)CHF₂, CF₃CF(CH₂F)C(O)CH₂F, andCF₃CF(CH₂F)C(O)CF₃. Some hydrofluoroketones can be prepared by reactinga fluorinated acid with a Grignard reagent such as an alkylmagnesiumbromide in an aprotic solvent, as described in Japanese Patent No.2,869,432. For example CF₂CF₂C(O)CH₃ can be prepared by reactingpentafluoropropionic acid with magnesium methyl bromide in dibutylether. Other hydrofluoroketones can be prepared by reacting a partiallyfluorinated acyl fluoride with hexafluoropropylene in an anhydrousenvironment in the presence of fluoride ion at elevated temperature, asdescribed in U.S. patent application Ser. No. 09/619306 (hereinincorporated by reference). For example, HCF₂CF₂C(O)CF(CF₃)₂ can beprepared by oxidizing tetrafluoropropanol with acidic dichromate, thenreacting the resulting HC₂H₄COOH with benzotrichloride to formHC₂H₄C(O)Cl, converting the acyl chloride to the acyl fluoride byreaction with anhydrous sodium fluoride, and then reacting theHC₂H₄C(O)F with hexafluoropropylene under pressure.

The gaseous mixture that comprises a fluorocarbon selected from thegroup consisting of perfluoroketones and hydrofluoroketones furthercomprises a carrier gas or carrier gases. Some possible carrier gasesinclude air, CO₂, argon, nitrogen and mixtures thereof. Preferably, thecarrier gas that is used with the perfluroketones is dry air.

The gaseous mixture comprises a minor amount of the fluorocarbon and amajor amount of the carrier gas. Preferably, the gaseous mixtureconsists of less than about 1% of the fluorocarbon and the balancecarrier gas. More preferably, the gaseous mixture contains less than0.5% by volume (most preferably less that 0.1% by volume) fluorocarbon,selected from the group consisting of perfluoroketones,hydrofluoroketones and mixtures thereof.

In order to keep the protective layer on the magnesium, the gaseousmixture is continuously, or nearly continuously, fed to the surface ofthe magnesium. Small breaks in the thin protective layer can then behealed without the possibility of such small breaks exposing moltenmagnesium to the air and initiating a fire.

A cover gas composition is of low toxicity both as it is applied to themolten magnesium and as it is emitted from the process in which it isused. Cover gases comprising low toxicity hydrofluoroketones andperfluoroketones, and mixtures thereof, will be safe mixtures as appliedto magnesium. However, all fluorine containing cover gas compositionproduce measurable amounts of hydrogen fluoride upon contact with themolten magnesium due to some level of thermal degradation and reactionwith magnesium at temperatures of 650 to 800° C. Hydrogen fluoride iscorrosive and toxic and its concentration in the emitted gas should beminimized. A preferred cover gas composition will, therefore, produceminimal hydrogen fluoride. See Examples, below.

Atmospheric lifetimes and global warming potentials for severalfluorocarbons used in accordance with this invention, along withcompounds currently known to be useful in the protection of moltenmagnesium as comparative examples, are presented in TABLE 2.

TABLE 2 Atmospheric Global Warming GWP Lifetime Potential (GWP) relativeCompound (years)⁽¹⁾ (100 year ITH)⁽¹⁾ to SF₆ HydrofluoroketoneHCF₂CF₂C(O)CF(CF₃)₂ ≦0.1⁽⁴⁾ ≦10⁽⁴⁾ 0.0005 PerfluoroketoneC₂F₅C(O)CF(CF₃)₂ 0.02⁽²⁾ 1⁽²⁾ 0.00005 Comparative Compounds:Hydrofluorocarbons FCH₂CF₃ 13.6 1600 0.07 CF₃CHFCHFCF₂CF₃ 17.1 1700 0.08CF₃CHFCF₃ 36.5 3800 0.17 HCF₂CF₃ 32.6 3800 0.17 SegregatedHydrofluoroethers C₄F₉OCH₃ 5.0 390 0.02 C₄F₉OC₂H₅ 0.8 55 0.002C₃F₇CF(OC₂H₅)CF(CF₃)₂ 2.5⁽²⁾ 210⁽²⁾ 0.01 Non-SegregatedHydrofluoroethers HCF₂OCF₂CF₂OCF₂H 7⁽³⁾ 1725⁽³⁾ 0.08 HCF₂OCF₂OC₂F₄OCF₂H7.1⁽³⁾ 1840⁽³⁾ 0.08 Other Fluorochemicals SF₆ 3200 22200 1.00 NF₃ 74010800 0.49 CClF₂CClF₂ 300 9800 0.44 CF₄ 50000 5700 0.26 C₂F₆ 10,00011,400 0.51 ⁽¹⁾World Meterological Organization Global Research andMonitoring Project-Report No. 44, “Scientific Assessment of OzoneDepletion: 1998, Vol. 2,” Chapter 10, Table 10-8, pp. 10.27 to 10.28.⁽²⁾Unpublished data, 3M Company, St. Paul, MN. ⁽³⁾Marchionni, G., etal., Journal of Fluorine Chemistry, 95, (1999), 41-50. ⁽⁴⁾Estimated, asdescribed below.

The perfluoroketones and hydrofluoroketones used in accordance with theinvention have much lower global warming potential (GWP) than thefluorocarbons known in the art such as SF₆, hydrofluorocarbons, andhydrofluoroethers. As used herein, “GWP” is a relative measure of thewarming potential of a compound based on the structure of the compound.The GWP of a compound, as defined by the Intergovernmental Panel onClimate Change (IPCC) in 1990 and updated in Scientific Assessment ofOzone Depletion: 1998 (World Meteorological Organization, ScientificAssessment of Ozone Depletion: 1998, Global Ozone Research andMonitoring Project—Report No. 44, Geneva, 1999), is calculated as thewarming due to the release of 1 kilogram of a compound relative to thewarming due to the release of 1 kilogram of CO₂ over a specifiedintegration time horizon (ITH).${{GWP}_{x}\left( t^{\prime} \right)} = \frac{\int_{0}^{ITH}{F_{x}C_{ox}^{{{- t}/\tau}\quad x}{t}}}{\int_{0}^{ITH}{F_{{CO}_{2}}{C_{{CO}_{2}}(t)}{t}}}$

where F is the radiative forcing per unit mass of a compound (the changein the flux of radiation through the atmosphere due to the IR absorbanceof that compound), C is the atmospheric concentration of a compound, τis the atmospheric lifetime of a compound, t is time and x is thecompound of interest.

The commonly accepted ITH is 100 years representing a compromise betweenshort-term effects (20 years) and longer-term effects (500 years orlonger). The concentration of an organic compound, x, in the atmosphereis assumed to follow pseudo first order kinetics (i.e., exponentialdecay). The concentration of CO₂ over that same time intervalincorporates a more complex model for the exchange and removal of CO₂from the atmosphere (the Bern carbon cycle model).

Carbonyl compounds such as aldehydes and ketones have been shown to havemeasurable photolysis rates in the lower atmosphere resulting in veryshort atmospheric lifetimes. Compounds such as formaldehyde,acetaldehyde, propionaldehyde, isobutyraldehyde, n-butyraldehyde,acetone, 2-butanone, 2-pentanone and 3-pentanone have atmosphericlifetimes by photolysis ranging from 4 hours to 38 days (Martinez, R.D., et al., 1992, Atmospheric Environment, 26, 785-792, and Seinfeld, J.H. and Pandis, S. N., Atmospheric Chemistry and Physics, John Wiley &Sons, New York, p. 288, 1998). CF₃CF₂C(O)CF(CF₃)₂ has an atmosphericlifetime of approximately 5 days based on photolysis studies at 300 nm.Other perfluoroketones and hydrofluoroketones show similar absorbancesnear 300 nm and are expected to have similar atmospheric lifetimes.

The very short lifetimes of the perfluoroketones and hydrofluoroketoneslead to very low GWPs. A measured IR cross-section was used to calculatethe radiative forcing value for CF₃CF₂C(O)CF(CF₃)₂ using the method ofPinnock, et al. (J. Geophys. Res., 100, 23227, 1995). Using thisradiative forcing value and the 5-day atmospheric lifetime the GWP (100year ITH) for CF₃CF₂C(O)CF(CF₃)₂ is 1. Assuming a maximum atmosphericlifetime of 38 days and infrared absorbance similar to that ofCF₃CF₂C(O)CF(CF₃)₂ the GWP for HCF₂CF₂C(O)CF(CF₃)₂ is calculated to be9. The perfluoroketones and hydrofluoroketones of the inventiontypically have a GWP less than about 10.

As a result of their rapid degradation in the lower atmosphere, theperfluoroketones and hydrofluoroketones have short lifetimes and wouldnot be expected to contribute significantly to global warming. The lowGWP of the perfluoroketones make them well suited for use as anenvironmentally preferred cover gas.

Also, the PFKs and HFKs of this invention can react more fully withmolten magnesium than does SF₆. As a result less unreacted cover gas canbe emitted to the atmosphere; less cover gas can be required to producea comparably performing protective film; or both. Consequently, usefulconcentrations of the cover gas can be lowered, thus reducing the globalwarming impact. The full substitution of fluorocarbons of the presentinvention for SF₆ can be accomplished without increasing the risk toworker safety since these materials (PFKs, and HFKs) are of lowtoxicity, are non-flammable, and are generally very innocuous materials.

Substitution for SF₆ with a PFK, or HFK, alone or as a mixture thereof,can provide protection of molten magnesium in various processes, such asmagnesium refining, alloying, formation of ingots or casting of parts.This substitution can be straightforward and can provide the sameutility as a reactive cover gas that only SF₆ does currently. Surfacefilms produced with the fluorocarbons of the present invention can bemore stable to higher temperatures than those formed with SO₂, enablingwork with higher melt temperatures (e.g., additional alloys, morecomplex casting parts). Improvements realized through the use offluorocarbons of the present invention as reactive cover gases caninclude a significant reduction in the emission of a potent greenhousegas (i.e., SF₆), a potential reduction in the amount offluorine-containing reactive cover gas required to provide protection,and a reduction in total emissions. This substitution can be donewithout increasing risks for workers since the fluorocarbons of thepresent invention are all safe materials with which to work, have lowtoxicity, are nonflammable, and are not a detriment to productionequipment.

The use of perfluoroketones, or hydrofluoroketones, or mixtures thereof,in a gaseous mixture demonstrate the ability to also put out fires thatare already occurring on the surface of molten magnesium. Therefore, thegases also may be used to extinguish fires on molten magnesium.

As discussed above, the use of a gaseous mixture comprising afluorocarbon selected from the group consisting of perfluoroketones,hydrofluoroketones, and mixtures thereof as a cover gas for handlingmolten magnesium instead of cover gases such as SF₆ provides anopportunity to reduce the emission of undesirable pollutants whileproducing similar, even increased amounts of magnesium. Accordingly, onecan use the present invention to produce protected magnesium or otherreactive metal or alloy and receive allocation of pollution credits.

In some applications, a magnesium producer can convert a facility whichutilizes cover gas comprising SF₆ to instead utilize a gaseous mixturecomprising a fluorocarbon selected from the group consisting ofperfluoroketones, hydrofluoroketones, and mixtures thereof as a covergas. Pollution credits may be allocated according to a function of: (1)how much protected reactive metal or alloy is processed or produced; (2)how much of a reduction in emissions or use of higher GWP cover gas(e.g., SF₆) is achieved; or (3) any other recognized system. As usedherein, “allocation” of pollution credits is meant to include any systemwherein credits are awarded, assigned, designated, or otherwise creditedby any public or private agency for the processing of reactive metals oralloys.

EXAMPLES

The present invention is further illustrated, but is not meant to belimited by, the following examples. The standard test procedure forevaluating the efficiency of each test fluorocarbon cover gas is givenbelow.

An approximately 3 kg sample of pure magnesium was placed in acylindrical steel crucible having an 11.4 cm internal diameter and washeated to 680° C. Cover gas was continuously applied to the 410 cm²surface of the molten magnesium through a 10 cm diameter ring formed of95 mm diameter stainless steel that was placed about 3 cm over themolten magnesium. The tubing was perforated on the side of the ringfacing the molten magnesium so that the cover gas flowed directly overthe molten magnesium. A square 20 cm×20 cm, 30 cm high stainless steelchamber with an internal volume of about 10.8 liters was fitted over thecrucible to contain the cover gas. The top of the chamber was fittedwith two 8.9 cm diameter quartz viewing ports and ports for a skimmingtool and thermocouple. A cover gas inlet, two gas sampling ports and adoor for adding fresh magnesium and for removing dross from the chamberwere placed on the sides of the chamber.

A stream of the cover gas was pumped from the chamber into the flow cellof an FTIR spectrophotometer (Midac I2000 Gas Phase FTIR) with a mercurycadmium telluride (MCT) detector. Using Modified Extractive FTIR (EPAMethod 320), the volumetric concentration of HF and the test cover gas(in ppmV) were measured continuously during experimentation. Once themixtures had stabilized, concentrations were measured over a period of 5to 10 minutes, average values of these concentrations were calculated,and those average values were used to make a relative comparison of thetest cover gases.

In all cases, initial magnesium melting was done using a standard covergas of 0.5% SF₆ in CO₂ at a flow rate of 5.9 L/min. The experimental gasmixture was then substituted for the standard cover gas mixture byutilizing a train of rotameters and valves. Dry air (having a −40° C.dew point) at a flow rate of 5.9 L/min was used to create the test covergas by evaporating a flow of test fluid in it such that a volumetricconcentration of 0.03 to 1 volume % fluorocarbon in air was produced.

During testing, the molten magnesium was observed for a period of about20 to 30 minutes (equivalent to 10 to 15 chamber volumes exchanges ofcover gas) to monitor any visible changes to the surface that wouldindicate the start of magnesium burning. The existing surface film wasthen removed by skimming the surface for about 3-5 minutes. The newsurface film that formed was then observed for a period of at 15-30minutes

The concentration of the fluorocarbon component of the cover gas mixturewas started at about 1% by volume in air and reduced sequentially insteps of ½ the previous concentration to a minimum fluorocarbonconcentration of 0.03 to 0.06%.

Comparative Example C1

C₄F₉OCH₃ (methoxy nonafluorobutane), a hydrofluoroether, has beendescribed as an effective fluorocarbon cover gas for molten magnesium inWorld Published Application WO 00/64614 (Example 5). In this comparativeexample, C₄F₉OCH₃ (available as NOVEC™ HFE-7100 Engineering Fluid from3M Company, St. Paul, Minn.) was evaluated as a fluorocarbon cover gasat 1% and at decreasing volumetric concentrations in air. In all cases,the volumetric flow rate for the cover gas/air mixture was 5.9 L/min. Atnominal concentrations of about 1, 0.5, 0.25 and 0.125% (corresponds to10000, 5000, 2500 and 1250 ppmV, respectively), C₄F₉OCH₃ produced a thinflexible surface film on molten magnesium immediately after skimming sothat no evidence of metal burning was observed. When the concentrationof C₄F₉OCH₃ was reduced to 0.0625% (i.e., 625 ppmV), some evidence ofburning was observed on the molten magnesium surface as white blooms,but no fire resulted. Exposure to fresh molten magnesium during skimmingcaused the HF concentration to remain essentially unchanged or to beincreased at all volumetric concentrations of C₄F₉OCH₃ tested.

The HF concentrations measured at the various volumetric concentrationsof C₄F₉OCH₃ tested are presented in TABLE 3.

TABLE 3 Concentration of Concentration of Concentration of C₄F₉OCH₃ inAir Hydrogen Fluoride over Hydrogen Fluoride over Over Molten StableSurface Molten Fresh Molten Magnesium Magnesium Film Magnesium Film (ppmby volume) (ppm by volume) (ppm by volume) 8300 4500 4100 4100 2000 22002000 980 1000 800 590 480

The data in TABLE 3 show that significant hydrogen fluoride is producedat 800 ppm volumetric concentration of C₄F₉OCH₃ (i.e., 480-590 ppm HF),the minimum concentration required to protect molten magnesium fromignition.

Example 1

CF₃CF₂C(O)CF(CF₃)₂(1,1,1,2,4,4,5,5,5-nonafluoro-2-trifluoromethyl-pentan-3-one), aperfluoroketone, was evaluated as a cover gas to protect moltenmagnesium from ignition using essentially the same procedure asdescribed in Comparative Example C1 using C₄F₉OCH₃. TheCF₃CF₂C(O)CF(CF₃)₂ was prepared and purified using the followingprocedures.

Into a clean dry 600 mL Parr reactor equipped with stirrer, heater andthermocouple were added 5.6 g (0.10 mol) of anhydrous potassium fluorideand 250 g of anhydrous diglyme (anhydrous diethylene glycol dimethylether, available from Sigma Aldrich Chemical Co.). The anhydrouspotassium fluoride was spray dried, stored at 125° C. and ground shortlybefore use. The contents of the reactor were stirred while 21.0 g (0.13mol) of C₂F₅COF (approximately 95.0 percent purity) was added to thesealed reactor. The reactor and its contents were then heated, and whena temperature of 70° C. had been reached, a mixture of 147.3 g (0.98mol) of CF₂═CFCF₃ (hexafluoropropylene) and 163.3 g (0.98 mol) ofC₂F₅COF was added over a 3.0 hour time period. During the addition ofthe hexafluoropropylene and the C₂F₅COF mixture, the pressure wasmaintained at less than 95 psig (7500 torr). The pressure at the end ofthe hexafluoropropylene addition was 30 psig (2300 torr) and did notchange over the 45-minute hold period. The reactor contents were allowedto cool and were one-plate distilled to obtain 307.1 g containing 90.6%CF₃CF₂C(O)CF(CF₃)₂ and 0.37% C₆F₁₂ (hexafluoropropylene dimer) asdetermined by gas chromatography. The crude fluorinated ketone waswater-washed, distilled, and dried by contacting with silica gel toprovide a fractionated fluorinated ketone of 99% purity and containing0.4% hexafluoropropylene dimers.

A sample of fractionated CF₃CF₂C(O)CF(CF₃)₂ made according to theabove-described procedure was purified of hexafluoropropylene dimersusing the following procedure. Into a clean dry 600 mL Parr reactorequipped with stirrer, heater and thermocouple were added 61 g of aceticacid, 1.7 g of potassium permanganate, and 301 g of the above-describedfractionated1,1,1,2,4,4,5,5,5-nonafluoro-2-trifluoromethyl-pentan-3-one. The reactorwas sealed and heated to 60° C., while stirring, reaching a pressure of12 psig (1400 torr). After 75 minutes of stirring at 60° C., a liquidsample was taken using a dip tube, the sample was phase split and thelower phase was washed with water. The sample was analyzed usinggas-liquid chromatography (“glc”) and showed undetectable amounts ofhexafluoropropylene dimers and small amounts of hexafluoropropylenetrimers. A second sample was taken 60 minutes later and was treatedsimilarly. The glc analysis of the second sample showed no detectabledimers or trimers. The reaction was stopped after 3.5 hours, and thepurified ketone was phase split from the acetic acid and the lower phasewas washed twice with water. 261 g of CF₃CF₂C(O)CF(CF₃)₂ was collected,having a purity greater than 99.6% by glc and containing no detectablehexafluoropropylene dimers or trimers.

The perfluorinated ketone, CF₃CF₂C(O)CF(CF₃)₂, was then evaluated as afluorocarbon cover gas at 1% and at decreasing volumetric concentrationsin air (i.e., at about 1.0, 0.5, 0.25, 0.12, 0.06 and 0.03% by volume;corresponds to 10000, 5000, 2500, 1250, 600 and 300 ppm, respectively).At all concentrations tested, CF₃CF₂C(O)CF(CF₃)₂ produced a thinflexible surface film on the molten magnesium during skimming andprevented metal ignition. The film visually appeared to be thinner andmore elastic than the surface film produced in the initial moltenmagnesium protection using SF₆ as a cover gas and in Comparative ExampleC1 using C₄F₉OCH₃ as a cover gas. The silvery-gray film produced wasstable and did not change appearance over at least 30 minutes. This isin contrast to the series using C₄F₉OCH₃, where evidence of metalburning was noted when the cover gas concentration was reduced to about625 ppm.

The HF concentrations measured at the various volumetric concentrationsof CF₃CF₂C(O)(CF₃)₂ tested are, presented in TABLE 4.

TABLE 4 Concentration of Concentration of Concentration ofCF₃CF₂C(O)CF(CF₃)₂ Hydrogen Fluoride over Hydrogen Fluoride in Air overMolten Stable Surface Molten over Fresh Molten Magnesium Magnesium FilmMagnesium Film (ppm by volume) (ppm by volume) (ppm by volume) 10400 420670 4800 470 775 2400 360 640 1200 280 370 560 180 120 480 120 100 28040 40

The data in TABLE 2 show that, at equal volumetric concentrations,significant less hydrogen flouride is produced using CF₃CF₂C(O)CF(CF₃)₂compared to C₄F₉OCH₃ as a cover gas. For example, at 2000 ppm C₄F₉OCH₃,980 ppm of HF was produced over the stable surface film and 1000 ppm ofHF was produced over the fresh molten film. In contrast, at 2400 ppmCF₃CF₂C(O)CF(CF₃)₂ (a slightly higher fluorocarbon concentration), only360 ppm of HF was produced over the stable surface film and 640 ppm ofHF was produced over the fresh molten film.

In summary, the perfluorinated ketone outperformed the hydrofluoroetheras a cover gas for molten magnesium (i.e. protected the molten magnesiumat lower concentrations) and also generated less hydrogen fluoride as adegradation product upon exposure to the molten metal surface.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scope ofthis invention. Accordingly, it is to be understood that this inventionis not to be limited to the illustrative embodiments set forth herein,but is to be controlled by the limitations set forth in the followingclaims and any equivalents thereof.

What is claimed is:
 1. A method for generating pollution creditscomprising (a) treating molten reactive metal or alloy to protect saidmolten metal or alloy from reacting with oxygen in air by (1) providingmolten metal or alloy and (2) exposing said molten metal or alloy to agaseous mixture comprising a fluorocarbon selected from the groupconsisting of perfluoroketones, hydrofluoroketones, and mixtures thereofto yield protected metal or alloy having a protective film thereon and(b) taking allocation of pollution credits.
 2. The method of claim 1wherein said protected molten metal or alloy is selected from the groupconsisting of molten metal or alloy and solid metal or alloy.
 3. Themethod of claim 1 wherein said pollution credits are allocated accordingto a function of how much protected metal or alloy is processed.
 4. Themethod of claim 1 further comprising converting means for handlingmolten reactive metal or alloy employing SF₆ as a cover gas to means forhandling molten reactive metal or alloy employing a gaseous mixturecomprising a fluorocarbon selected from the group consisting ofperfluoroketones, hydrofluoroketones, and mixtures thereof.
 5. Themethod of claim 4 wherein said pollution credits are allocated accordingto a function of said reduction in SF₆ usage.
 6. The method of claim 1wherein said fluorocarbon is a hydrofluoroketone that is selected fromthe group consisting of HCF₂CF₂C(O)CF(CF₃)₂, CF₃C(O)CH₂C(O)CF₃,C₂H₅C(O)CF(CF₃)₂, CF₂CF₂C(O)CH₃, (CF₃)₂CFC(O)CH₃, CF₃CF₂C(O)CHF₂,CF₃CF₂C(O)CH₂F, CF₃CF₂C(O)CH₂CF₃, CF₃CF₂C(O)CH₂CH₃, CF₃CF₂C(O)CH₂CHF₂,CF₃CF₂C(O)CH₂CHF₂, CF₃CF₂C(O)CH₂CH₂F, CF₃CF₂C(O)CHFCH₃,CF₃CF₂C(O)CHFCHF₂, CF₃CF₂C(O)CHFCH₂F, CF₃CF₂C(O)CF₂CH₃,CF₃CF₂C(O)CF₂CHF₂, CF₃CF₂C(O)CF₂CH₂F, (CF₃)₂CFC(O)CHF₂,(CF₃)₂CFC(O)CH₂F, CF₃CF(CH₂F)C(O)CHF₂, CF₃CF(CH₂F)C(O)CH₂F,CF₃CF(CH₂F)C(O)CF₃, and mixtures thereof.
 7. The method of claim 1wherein the gaseous mixture further comprises a carrier gas.
 8. Themethod of claim 7 wherein said carrier gas is selected from the groupconsisting of air, CO₂, argon, nitrogen, and mixtures thereof.
 9. Themethod of claim 1 wherein said perfluoroketone is selected from thegroup consisting of CF₃CF₂C(O)CF(CF₃)₂, (CF₃)₂CFC(O)CF(CF₃)₂,CF₃(CF₂)₂C(O)CF(CF₃)₂, CF₃(CF₂)₃C(O)CF(CF₃)₂, CF₃(CF₂)₅C(O)CF₃,CF₃CF₂C(O)CF₂CF₂CF₃, CF₃C(O)CF(CF₃)₂, perfluorocyclohexanone, andmixtures thereof.